Pope Benedict XVI Pope Benedict XVI at St Mary's University College in south-west London. Six men were arrested today in an alleged plot against the pontiff.

Scotland Yard arrested six street cleaners today over an alleged terrorist threat to the pope.

The men were arrested by armed officers in central London at 5.45am on the second day of the pope's visit.

Sources say the men are believed to be Muslim and that some of them are Algerian. A spokesperson for Westminster city council said they were working for Veolia Environmental Services, a contractor which employs 650 on-street staff to keep Westminster's streets clean. The suspects, aged 26, 27, 29, 36, 40 and 50, were held under the Terrorism Act 2000 at business premises in the central London. They were taken to a nearby police station, where they will be interviewed by detectives.

An assessment of the intelligence judged it to be so severe that police needed to disrupt the alleged plot immediately. The first information was received yesterday evening, and officers moved into place during the night to make the arrests.

Searches were being carried out at business premises in central London and at residential premises in north and east London today.

Sources said the arrests were "precautionary" and detectives were still assessing the scale of the danger, if any, posed to the pope.

The terrorist threat level in the United Kingdom remained unchanged after the arrests, which came hours after the head of MI5, Jonathan Evans, warned of an attack against Britain.

A Scotland Yard spokesman said: "At approximately 5.45 this morning, five men were arrested by officers from the Metropolitan police service's Counter Terrorism Command on suspicion of the commission, preparation or instigation of acts of terrorism. Searches are being carried out under the Terrorism Act 2000 at business premises in central London and residential premises in north London and east London." The sixth man aged 29, was arrested on suspicion of the commission, preparation or instigation of acts of terrorism was taken from a home in north London and is being interviewed by detectives.

He added: "Initial searches have not uncovered any hazardous items. Today's arrests were made after the police received information. Following initial inquiries a decision was made to arrest the men.

"Following today's arrests policing arrangements for the papal visit were reviewed. We are satisfied that the current policing plan remains appropriate and the papal itinerary has not changed. There is no change to the UK threat level."

Leith Penny, Westminster city council's director for city management, said: "The UK's security is obviously of critical importance and can never be taken for granted, and so our congratulations must go to the police authorities for their swift action today.

"While police investigations are ongoing and no charges have been brought, we can confirm that the people at the centre of the investigation work for one of our contractors, Veolia Environmental Services, who employ 650 on-street staff to keep Westminster's streets clean and free from rubbish."

Penny said both the company and the council worked closely with the authorities to make sure that all those they employed underwent Home Office checks to assess their eligibility to work in the UK.

He added: "We are confident that these checks are robust and we will continue to work with the police and other authorities during this investigation."

Father Federico Lombardi, the Vatican's head of press, said that the visit would continue with "courage and joy" despite the arrests, adding: "We are calm … We are totally confident in the work of the police and Scotland Yard."

Lombardi's words were echoed by the Catholic Bishops' Conference of England and Wales, which said it had "absolute confidence" in the security operation to protect the pope and the public.

It described the visit as wonderful and noted that Benedict had been "warmly welcomed wherever he goes".

A huge security and public order operation swung into action yesterday as the pope touched down in Britain.

Thousands of officers are involved in the operation from forces including the Met, Strathclyde, Lothian and Borders, West Midlands and British transport police.

The cost of policing the pope's visit could reach £1.5m and is being co-ordinated by South Yorkshire's chief constable, Meredydd Hughes.

Senior officers said last week that there was no information ahead of the visit to suggest any specific group wanted to attack the pope. But they warned people not to underestimate the "passion and the fervour" the visit would evoke.

Police also interviewed mentally unstable people who they feared could pose a threat to the pope.

The pontiff is also protected by members of the Swiss Guard. He often travels in the popemobile, which has a bulletproof glass enclosure.

The Vatican abandoned the use of an open-topped vehicle after the assassination attempt on Pope John Paul II in 1981.